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Baby photos of distant planet are first to capture formation

By Jacob Aron

Aww, who’s a widdle planet, then? Astronomers have got their first glimpse at growing baby worlds around a distant star. Watching them develop into strapping Jupiters should give us a better understanding of how planetary systems form.

We’ve found nearly 2000 exoplanets, but so far planetary formation has only been seen indirectly by studying gaps in large discs of dust and gas around stars. Now Stephanie Sallum of the University of Arizona, Tucson, and her colleagues have spotted direct signatures of a young trio of protoplanets bulking up.

Gathering dust

The team used the Magellan Adaptive Optics System in Atacama, Chile, and the Large Binocular Telescope in Arizona to study a star called LkCa 15, which is only two million years old – a cosmic blink of an eye. It is surrounded by a doughnut-shaped dust and gas disc, and observations in 2009 had already suggested three new-born planets are forming in the central hole.

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Now the team has confirmed those planets exist by tracing their orbits some years later, and found that all of them are around the mass of Jupiter. What’s more, they have detected signals that could only come from an actively growing world.

The team looked at the signatures these planets give off in two infrared wavelengths and one particular visible wavelength called H-alpha, which is emitted by glowing hydrogen gas. The innermost planet, LkCa 15 b, showed up in all wavelengths, and the H-alpha signal suggests that hot gas, at around 10,000 °C, is falling on to the planet as it grows. Planet c was only seen in infrared, suggesting it is sucking up less gas, while d only showed up faintly at one infrared wavelength, making it hard to draw any firm conclusions.

Still, seeing even just one example of an accreting planet should prove very useful. “Now we’ve found a system where we can go look in depth to try to understand the details of planet formation,” says Sallum. “One interesting question is whether accretion on to planets is steady or variable. That’s something future observations would help us get a feel for.”